1. Field of the Invention
This invention pertains in general to X-ray systems and in particular to a method and system for collimating the X-rays in such systems.
2. Background of the Invention
Since the discovery of X-rays in 1895 by Wilhelm Roentgen, the predominant method for capturing an X-ray image has been by exposing a photographic film. A disadvantage of using photographic film is that chemical processing must be performed on it to convert the latent X-ray image into a viewable image. Because of this chemical processing, there is a delay between when the film is exposed and when the image is viewable. For medical X-ray images taken in the emergency room, for example, this delay in viewing the image can be critical. Chemical processing of the film, moreover, requires special handling and disposal of the chemicals in order to avoid environmental contamination.
In recent years, computed radiography (CR) has provided a means to take X-ray images without using photographic film or chemical processing. When using CR, the X-ray image is captured with a photostimulable luminescent plate. The plate is then placed in a special scanner to convert the captured image into a digital form for subsequent viewing at a computer workstation. Though this technique eliminates the use of photo processing chemicals, there is still a significant delay introduced before the X-ray image is viewable.
More recently, a technique known as digital radiography (DR) has been developed that eliminates both the need for chemical processing and the significant delay in producing a viewable image. In DR, a sensor unit, typically an array of amorphous silicon, is used to capture the X-ray image and produce a digital representation of the image. The sensor unit is coupled to a control station with a cable or other communications link. The cable provides power to the sensor unit and transmits digital communication signals between the sensor unit and the control station. Accordingly, the control station receives substantially real-time data describing the X-rays detected by the sensor plate.
In all of the above X-ray systems, there is a need to aim the X-ray generator and collimate the X-rays before capturing the image. Typically, the generated X-rays pass through an adjustable collimator having an aperture that restricts the size and shape of the primary beam of X-ray radiation. Collimation serves to: 1) reduce scatter from X-rays not required for imaging, thereby improving image quality; and 2) reduce unnecessary X-ray exposure to the patient. Before exposing the X-ray image, an X-ray technologist manually adjusts the collimator by shining a light located at or near the X-ray generator onto the X-ray sensor or the patient""s anatomical region of interest. The technologist observes the light reflecting off the sensor plate or patient and manually adjusts the aperture in the collimator. Once the light is properly collimated, the X-ray image is exposed.
In a fast-paced medical environment, such as a hospital emergency room or a busy clinic, the technologist wastes valuable time and resources when visually collimating the X-ray generator. Moreover, locating the collimating light near the generator adds an extra level of complexity to the generator design. Therefore, there is a need for an X-ray system that simplifies the collimation process. Preferably, the system would reduce the time and effort expended by the X-ray technologist to collimate the X-rays and would not need a visible light to perform collimation.
The above needs are met by a digital X-ray system (100) having a generator (108) and a sensor unit (110) in communication with the control station (112). In addition to the sensor unit (110) and control station (112), the digital X-ray system (100) preferably includes a preview monitor and operation panel (114) for controlling the X-ray system (100), an image archiver (116) for storing images captured by the sensor unit (110), a viewing workstation (118) for viewing and manipulating the images stored in the archiver (116), and a hard copy output device (120) for printing the images. In a preferred embodiment of the present invention, the sensor unit (110) captures the digital X-ray images and transmits the images to the control station (112). Then, the images can be manipulated by the other components of the X-ray system (100).
The generator (108) preferably includes a radiation source such as an X-ray tube (210) and a collimator (212). The collimator (212) includes a portion blocking the radiation, an adjustable aperture (226) through which a primary beam of radiation passes, and an actuator (230) for adjusting the aperture. By adjusting collimation parameters which affect the size and shape of the aperture (226), the size and shape of the primary beam can be adjusted.
The sensor unit (110) is surrounded by a protective cover (310). Within the cover (310) are preferably a scintillator (312), a sensor plate (314), and sensor electronics (316). When the sensor unit (110) is exposed to X-rays, the scintillator (312) converts the X-rays into visible light. The position and intensity of the light is detected by the sensor plate (314) and stored as a digital image. The digital image is then transmitted to the control station (112).
In use, the aperture (226) is constricted (410) and a relatively short duration beam is directed (412) at the sensor unit (110). The resulting digital image is transmitted (416) to the control station (112). This process is repeated one or more times with a displaced aperture (226) and the control station (112) calculates (428) a calibration coefficient from the captured digital images. This coefficient is preferably stored (430) in the control station (112).
To capture an X-ray image of a subject, a reference image is captured (514) from a short-duration beam and transmitted (516) to the control station (112). The control station (112) analyzes (518) the reference image and uses the calibration coefficient to adjust the aperture (226) to achieve a desired goal. In one embodiment, the aperture (226) is adjusted to cover the entire field of the sensor plate (314). In another embodiment, the aperture (226) is adjusted to restrict the X-ray beam and eliminate unwanted exposure beyond the periphery of the X-ray subject. In yet another embodiment, the technologist views the digital image and defines an area of exposure. The control station (112) automatically calculates aperture (226) parameters to restrict the beam to precisely the defined area. Once the aperture (226) is adjusted, the image of the X-ray subject is captured (526).